Various well logging techniques are known in the field of hydrocarbon exploration and production. These techniques use instruments or tools equipped with transmitters adapted to emit energy into a subsurface formation that has been penetrated by a borehole. In this description, “instrument” and “tool” will be used interchangeably to indicate, for example, an electromagnetic instrument (or tool), a wireline tool (or instrument), or a logging-while-drilling tool (or instrument). The emitted energy interacts with the surrounding formation to produce signals that are then detected and measured by one or more sensors. By processing the detected signal data, a profile of formation properties can be generated.
Electromagnetic logging tools, including electromagnetic induction and wave propagation logging tools, are used to determine the electrical properties of formations surrounding a borehole. Such logging tools obtain measurements relating to the resistivity (or its inverse, conductivity) of the formation that, when interpreted, allow one to infer various petrophysical properties of the formation and fluids therein. The physical principles of electromagnetic induction resistivity well logging are well known.
Electromagnetic logging tools use transmitter and receiver antennas. In some embodiments, such antennas may be operable as a transmitter and/or a receiver. Those skilled in the art will appreciate that an antenna may be used as a transmitter at one instant and as a receiver at another. It will also be appreciated that the transmitter-receiver configurations disclosed herein are interchangeable due to the principle of reciprocity, i.e., the “transmitter” may be used as a “receiver”, and vice-versa.
Conventional electromagnetic logging tools employ axial transmitter and receiver antennas having magnetic dipole moments substantially along the longitudinal axis of the tool. Such tools do not have azimuthal sensitivity. In high angle or horizontal wells, measurements obtained with axial antennas do not contain information about the directionality of the formation that allows distinguishing whether the borehole is approaching, for example, an electrically conductive layer from above or below. Such information is used, for example, in well placement applications. Logging tools comprising one or more antennas having a magnetic dipole moment tilted or transverse with respect to the tool axis, such as those described, for example, in U.S. Pat. Nos. 5,508,616, 6,163,155, 6,476,609, 7,656,160, 8,466,683, 7,755,361, U.S. Pat. Pub. No. 20140292340, and U.S. Pat. No. 9,389,332 have been proposed. Such logging tools can provide a directional measurement containing information about the directionality of the formation. It further provides more information used for various formation evaluation applications.
FIG. 1 illustrates a prior art drilling system 100. The drilling system 100 shown includes a drilling rig 10 positioned over a wellbore 11. A drilling tool assembly, which may include a drill string 12 and a drill bit 15 coupled to the lower end of drill string 12, may be disposed in wellbore 11. Drill string 12 and bit 15 may be rotated by a kelly 17 coupled to the upper end of drill string 12. The kelly 17 may be rotated by engagement with a rotary table 16 disposed on the drilling rig 10. The kelly 17 and the drill string 12 may be suspended by a hook 18 coupled to the kelly 17 by a rotatable swivel 19.
Drilling fluid may be stored in a pit 27 and may be pumped through the center of drill string 12 by a mud pump 29 to flow downwardly (shown by arrow 9). After circulation through bit 15, the drilling fluid may circulate upwardly (indicated by arrow 32) through an annular space between wellbore 11 and drill string 12. Flow of the drilling mud may lubricate and cool bit 15 and lift drill cuttings made by bit 15 to the surface for collection and disposal.
A bottom hole assembly (BHA) 110 may be connected to drill string 12. The bottom hole assembly 110 may include a stabilizer 140 and a sub 130, which may be coupled to a local measuring device 120. The bottom hole assembly 110 may also include a downhole communications system 150, such as a pressure modulation (mud pulse) telemetry system. Pressure modulation telemetry can include various techniques for selectively modulating the flow (and consequently the pressure) of the drilling mud. The drill string 12 can also (or alternatively) comprise wired drill pipes that support high-speed telemetry between the surface and downhole tools.
A transducer 31 disposed at the earth's surface may be configured to detect pressure variations and to conduct signals to an uplink telemetry subsystem 90 for demodulation and interpretation. The demodulated signals may be forwarded to a processor 85 and a recorder 45 for further processing and/or storage. The surface equipment may optionally include a downlink telemetry subsystem 95, which may include a pressure modulation transmitter (not detailed) that can modulate the pressure of the drilling mud circulating downwardly to communicate control signals to the bottom hole assembly 110. It should be understood that the mud telemetry described above is one example of a communication means. Other telemetry systems known in the art may also be used.
The downhole communications system 150 may also include various types of processors and controllers (not shown) for controlling the operation of sensors disposed therein, for communicating command signals to the local measuring device 120, and for receiving and processing measurements transmitted from the local measuring device 120. Sensors in bottom hole assembly 110 and/or downhole communications system 150 may include magnetometers, accelerometers, gyro-meters, and the like.
Various instruments disposed in the bottom hole assembly 110, the downhole communications system 150, and the local measuring device 120 may be referred to collectively as a logging-while-drilling (LWD) tool or measurement-while-drilling (MWD) tool. The bottom hole assembly 110, the processor 85, and/or the downhole communications system 150 may include various forms of data storage or memory that can store measurements made by any or all of the sensors, including sensors disposed in the local measuring device 120, for later processing during or after the drill string 12 is withdrawn from wellbore 11. In one implementation, the LWD or MWD tool may be a propagation resistivity tool. As such, the bottom hole assembly 110, the downhole communications system 150, and the local measuring device 120 may be used to obtain well log data obtained from various transmitter-receiver pairs (to be discussed further below) and at multiple frequencies. Such data may be communicated to the surface while the LWD or MWD tool is downhole or stored in the LWD or MWD tool for later readout.